Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, middle distillates, naphtha, and gasoline boiling hydrocarbons among others by hydroprocessing a hydrocarbon feed stock derived from crude oil or heavy fractions thereof. Hydroprocessing can include, for example, hydrocracking, hydrotreating, hydrodesulphurization and the like. Feed stocks subjected to hydroprocessing can be vacuum gas oils, heavy gas oils, and other hydrocarbon streams recovered from crude oil by distillation. For example, a typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.) and usually at least about 50 percent by weight boiling above 371° C. (700° F.), and a typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydroprocessing is a process that uses a hydrogen-containing gas with suitable catalyst(s) for a particular application. In many instances, hydroprocessing is generally accomplished by contacting the selected feed stock in a reaction vessel or zone with the suitable catalyst under conditions of elevated temperature and pressure in the presence of hydrogen as a separate phase in a three-phase system (i.e., hydrogen gas, a liquid hydrocarbon stream, and a solid catalyst). Such hydroprocessing systems are commonly undertaken in a trickle-bed reactor where the continuous phase throughout the reactor is gaseous.
In the trickle-bed reactor, a substantial excess of the hydrogen gas is present. In many instances, a typical trickle-bed hydrocracking reactor requires up to about 10,000 SCF/B of hydrogen at pressures up to 17.3 MPa (2500 psig) to effect the desired reactions. In these systems, because the continuous phase throughout the reactor is a gas-phase, large amounts of excess hydrogen gas are generally required to maintain this continuous phase. However, supplying such large supplies of gaseous hydrogen at the operating conditions needed for hydroprocessing adds complexity and capital and operating expense to the hydroprocessing system.
In order to supply and maintain the needed amounts of hydrogen, the resulting effluent from the trickle-bed reactor is commonly separated into a gaseous component containing hydrogen and a liquid component. The gaseous component is directed to a compressor and then recycled back to the reactor inlet to help supply the large amounts of hydrogen gas needed to maintain the continuous gaseous phase therein. Conventional trickle-bed hydrocracking units typically operate up to about 17.3 MPa (2500 psig) and, therefore, require the use of a high-pressure recycle gas compressor in order to provide the recycled hydrogen at necessary elevated pressures. Often such hydrogen recycle can be up to about 10,000 SCF/B, and processing such quantities of hydrogen through a high-pressure compressor adds complexity, increased capital costs, and increased operating costs to the hydroprocessing unit. In general, the recycle gas compressor represents about 15 to about 30 percent of the cost of a hydroprocessing unit.
Many reactions undertaken using hydroprocessing reaction zones, such as hydrodesulfurization, hydroisomerization, hydrodenitrification, hydrodeoxygenation, hydrocracking, and aromatic saturation to suggest but a few are exothermic and, therefore, result in a temperature rise of the hydrocarbon stream across the catalyst reaction bed. In many of the reactions, such as hydroisomerization, hydrotreating petroleum fractions containing a lower concentration of heteroatoms, hydrocracking in a second stage after severe hydrotreatment, where the consumed hydrogen can be relatively low, between about 50 and about 500 SCF/B, and the reactions can result in heat releases causing a temperature increase in excess of about 28 to 56° C. (50 to 100° F.). In other reactions, such as hydrotreating petroleum fractions containing higher concentration of heteroatoms, full conversion hydrocracking in a single stage, aromatic saturation of a highly aromatic petroleum fraction, the consumed hydrogen can be higher than about 500 SCF/B, and the heat release from such reactions may cause temperature increases in excess of about 37° C. (100° F.). In still other reactions, such temperature increases can result in the temperature of the hydrocarbons exceeding about 399° C. (750° F.) to about 427° C. (800° F.), which is generally unacceptable for the catalysts used in these reactions. In typical trickle bed reaction zones, the large amounts of recycle gas introduced into the inlet of the reactor helps manage unacceptable reactor temperature increases.
In some cases, it is desired to eliminate the costly recycle gas compressor by using a two-phase hydroprocessing system (i.e., a liquid hydrocarbon stream and solid catalyst). In these reaction systems, the continuous phase throughout the reactor is liquid rather than gas and, therefore, generally do not need a source from a high pressure recycle gas compressor. Such two-phase systems generally use only enough hydrogen dissolved in the liquid-phase to saturate the liquid in the reactor. However, it can be more difficult to manage the temperature profile in such reactors. Diluents added as recycle liquids or quench streams, can help manage temperatures, but these solutions can reduce the effectiveness of the hydroprocessing reactions as they tend to reduce the contact time between the unconverted oil and the catalysts resulting in less effective conversions to other products. Such diluents also may introduce other materials with the process that impact reaction rates and other vessels parameters.